One thing I have learned in decades of backpacking is that to a point, the weight of an empty backpack should not be as important in pack selection as the suspension of the pack. I am an ounce counter but know that the lightest packs are generally not the best packs for carrying heavier loads. I still see folks on the trail with an old external pack frame with gear mounted to it. They stick to it and swear by it.
I think the heart of a good pack is the pack frame more even than the capacity and increasingly, I am coming to the same conclusion as two quality U.S. pack manufacturers. Both Kifaru (Colorado) https://store.kifaru.net/bikini-platform-frame-and-suspension-p116.aspx and Mystery Ranch (Montana) http://www.mysteryranch.com/Packs/Hunting/frame/NICE-Frame sell pack frames separate from a selection of various sized packs that attach to the frames. Each company has more than one frame to offer. I would rather carry 40 pounds in a heavier pack with good suspension than 25 pounds in a flimsy ultralight pack.
This comes to my second point of flexibility. This means that a person could have a single pack frame with several different sized packs that attach to it depending on the mission. Are you out for an overnight, three days or two weeks? A second feature of these packs is the Pouch Attachment Ladder System (PALS) webbing which allows for attaching external pouches pods and pockets using Modular Lightweight Load-carrying Equipment (MOLLE), For example this allows for specialized items like a telescopic camera lens being externally attached, protected, and providing quick access to it.
An example of this is my Kifaru Express pack that is a 2,300 Cubic Inch pack but because it has several attachment points, I added a zippered top pocket and two side pockets. With these additions, it is closer to 2,700 Cubic Inches. (It has been discontinued and is a stand-alone pack with it’s own harness and belt). You might argue that I should just buy a larger pack but I often use this pack as a daypack, by removing the added pockets and pouches.
This flexibility is important for search and rescue (SAR) folks like me that need to have an overnight pack and a three day bag packed and ready to go 24/7.
Another factor is durability. My Kifaru pack is made of 1,000d Cordura. 500d Cordura is available as a lighter option. I have owned my Express pack for about seven years and gone on scores of missions with it. Many searches were off trail and involved traveling through and crawling under Manzanita and Buckbrush. This would have torn other packs to shreds. The Kifaru and Mystery Ranch packs are expensive but over the lifespan of the pack, they are cost effective. Navigate on on EBay and see what kinds of price used packs from these companies are still worth.
Fit/Sizing is the final issue. Many packs come in only one size. These companies build the packs individually based on the measurements of the buyer. This assures proper fit and comfort.
I recently had some email correspondence with Patrick Smith (formerly owner of Mountainsmith Packs) who is uncompromising and innovative with his newer Kifaru Company. He has some exciting plans for the future and many will be watching.
In sum then, think about pack suspension, flexibility, durability and fit/sizing in addition to capacity when selecting a backpack. My next pack will start with a pack frame first.